The purpose of the research proposed in this application is to investigate the process of mapping sound to meaning in infant word learning. By the end of their first year, infants have developed important skills for word learning, including the ability to segment words from fluent speech and the ability to form associations between the sounds of words and their meanings. However, the nature of the relationship between these skills remains unclear. We hypothesize that once infants have segmented a word form from speech, it is rapidly available for linking to meaning. This project will test this hypothesis and examine whether there are developmental changes in infants' ability map meaning to newly segmented words. Additionally, an ongoing puzzle in the study of language acquisition is how to explain the wide variability in early vocabulary development. We propose that by examining individual differences in skills underlying word learning, we may contribute to the understanding of individual differences in vocabulary development. This project will examine how infants' comprehension and production vocabulary sizes relate to infants' performance with a set of interrelated basic word learning skills: segmenting words, mapping sound to meaning, and combining these two skills for mapping meaning to newly segmented words. By investigating whether these skills predict infants' current and future vocabulary size, this project will suggest possible sources of individual differences in language development and may indicate means of identifying infants at risk for significant language delays.